highspy: Highs.val should work for highs_linear_expression

[few]

In my modeling, I'm carrying around several linear expressions where sometimes I'm only interested in the value of the expressions and not the individual variables. It would be nice if that could be supported.

h = highspy.Highs()
x = h.addVariable(ub=3, obj=1)
y = h.addVariable(ub=4, obj=1)
h.maximize()

assert abs(h.val(x)-3) < 1e-6
assert abs(h.val(y)-4) < 1e-6
assert abs(h.val(x+y)-7) < 1e-6 #AttributeError: 'highs_linear_expression' object has no attribute 'index'

[mathgeekcoder]

This is a simple feature (if we limit to linear expressions). I've already added support in #1891. Will commit when I've finished with all the new tests and suitable highspy coverage.

Note: it also supports constant addition and all in/equality variants. Strict inequalities (<,>) aren't supported.

h.val(x - 3)
h.val(x + y - 7)
h.val(x + y <= 7)  # returns True/False
...

[jajhall]

Closed by #1942